(In describing a charge transfer device this application will use the convention of considering the surface of a semiconductor substrate on which the gate electrodes of the charge transfer device are disposed as its "top" surface, regardless of the actual orientation of the device in space; words such as "under" and "over" will be in accordance with this convention.)
Typically, a floating diffusion output stage incorporates a metal-insulator-semiconductor field effect transistor (MISFET) connected with gate electrode to the floating diffusion and operated in common-drain (or common-source) configuration as an electrometer to measure the potential on the floating diffusion. This potential is indicative of charge in a potential well "under" the floating diffusion. The measurement of potential is at signal-sampling intervals having interspersed amongst them reset intervals, during which reset intervals the floating diffusion is clamped by MISFET action to the reference potential at a reset drain. More particularly, the floating diffusion is a virtual source in this MISFET action, which occurs responsive to potential applied to a reset gate electrode between the floating diffusion and reset drain. It is standard practice to interpose a gate electrode between the floating diffusion and the reset electrode and to apply direct potential to this gate electrode so interposed, this being done to prevent potential responses to the reset pulses from appearing on the floating diffusion owing to electrostatic induction.
The resetting process of periodically clamping the floating diffusion to the potential at the reset drain is undesirably accompanied by a type of noise called "reset" noise, arising from variations in the potential left upon the floating diffusion from one reset interval to another. (Reset noise is a problem with charge transfer devices having floating gate output stages, as well as ones with floating diffusion output stages.) Reset noise is the predominant noise in the upper-video frequencies of the output signals of charge transfer devices such as CCD imagers, typically being about 8 dB larger than noise in the MISFET electrometer stage following the floating diffusion. At lower video frequencies flicker noise or "1/f" noise predominates.
The desirability of reducing both flicker noise and reset noise has led to the practice of correlated double sampling in which the signal on the floating diffusion is sampled, firstly, at a time when charge dependent upon reset noise but not upon signal is present in the potential well induced "under" the floating diffusion and, secondly, at a time when charge dependent upon both that reset noise and upon signal is present there. Each pair of samples is then differentially combined to generate samples which depend substantially only on the signal, with reset noise being suppressed. Correlated double sampling becomes less practical as the sampling rates of the charge transfer device output stage increases. Pulse widths become narrower and pulse spacing is lessened towards the limit allowed by the time for charge equilibration under the floating diffusion- or floating-gate output. As clock rates rise to more than a few megahertz, the correlated double sampling technique becomes progressively more difficult to employ.
L. N. Davy in his U.S. Pat. No. 4,330,753 issued May 18, 1982 and entitled "METHOD AND APPARATUS FOR RECOVERING SIGNAL FROM A CHARGE TRANSFER DEVICE" describes a method for obtaining what he characterizes as relatively noise-free information signals from the output stage of a charge transfer device. In the method Davy describes, the output signal from the regularly sampling electrometer stage is passed through a band-pass filter to separate double-sideband amplitude-modulation (DSB AM) sidebands flanking a harmonic of the clocking frequency of the electrometer stage. The separated sidebands are then synchronously detected using a switching demodulator operated at the harmonic of that clocking frequency. The amplitude-modulating signal is heterodyned to baseband spectrum by the switching demodulator. The baseband spectrum of the synchronously detected AM sidebands is separated from the harmonic spectra associated with it and is used as the output signal from the charge transfer device, rather than the baseband spectrum of the imager output signal, which is suppressed by the band-pass filtering before synchronous detection. The method Davy describes is effective in suppressing the 1/f noise in the electrometer stage, since 1/f noise resides principally in the baseband. It is relatively simple as compared with correlated double sampling to reduce the baseband entirely or at least up to the one or two megahertz frequencies where 1/ f noise exceeds the thermal noise background. On the other hand, while with correlated double sampling 20 dB noise reduction is obtainable at 100 kHz in the imager system the inventor has been working with, it is practically difficult to obtain more than three to six dB noise reduction at 1 MHz. Difficulties arise with making pulses narrower owing to system bandwidth limitations, or with making them closer together, owing to the time needed for charge equilibration under the floating diffusion.
Reset noise is ignored by Davy; but, as noted above, reset noise is a primary source of noise in a semiconductor imager with a floating gate or floating diffusion output stage. Reset noise is wideband and extends through the harmonic frequency spectra of the video samples supplied at the semiconductor imager output, so reset noise is a major contributor to noise, even when synchronous detection of the sidebands surrounding a clocking frequency harmonic is used to recover video signal from the imager output samples. (It is to be understood that reset noise does not refer to the simple feedthrough of reset pulses, the reduction of which feedthrough Davy does concern himself with.)
W. F. Kosonocky and J. E. Carnes of RCA Corporation's David Sarnoff Research Center described the floating-diffusion amplifier in their paper entitled "Basic Concepts of Charge-Coupled Devices" and published September 1975 in RCA Review, Vol. 36, pp. 566-593. The paper suggests resetting of the floating diffusion to the barrier potential afforded by a gate biased with direct potential and interposed between the floating diffusion and a gate operative as a reset gate. That is, the floating diffusion is reset to a channel potential within the charge transfer channel in which the floating diffusion is disposed, rather than to the drain potential at the end of the charge transfer channel. This approach to resetting the floating diffusion has for the most part been discarded as impractical by those skilled in the art, because it introduces a pronounced low-frequency distortion in the modulation transfer function (MTF). Smearing of the trailing edges of bright areas into darker areas is noted in television displays based on the video samples from CCD imagers having floating diffusion output stages reset to in-channel voltages rather than to drain voltages, when the output stages have their output samples processed conventionally, using a sample-and-hold output circuit to suppress clock feedthrough.
While operating a CCD imager with floating diffusion output stage connected by high-pass filter to a sample-and-hold circuit for suppressing clock feedthrough and for eliminating the baseband and its associated 1/f noise, the present inventor happened to misadjust the level of reset pulses to the output stage. Surprisingly, reset noise in the video signal from the sample-and-hold circuit fell to levels normally experienced only with correlated double sampling. It was determined that resetting was to an in-channel voltage, even though the television display based on the video from the imager did not exhibit the low-frequency distortion normally associated with such resetting operation.
The inventor, in his U.S. patent application Ser. No. 590,044, filed Mar. 15, 1984, entitled "CCD FLOATING-ELEMENT OUTPUT STAGES PROVIDING LOW RESET NOISE WITH SINGLE SAMPLING", and assigned like the present application to RCA Corporation, has described another way to keep reset noise low. This other way allows the floating diffusion to be reset by relatively low-impedance clamp to a reset drain potential, responsive to reset pulses being applied to the reset gate between the floating diffusion and the reset drain diffusion of the CCD output stage. A simple RC high-pass filter is used to differentiate the CCD output signals before their synchronous detection at the first harmonic of their clocking, or sampling, rate. The corner frequency of this RC high-pass filter is chosen to suppress the 1/f noise in the lower baseband frequencies of the signal synchronously detected. Reset noise is suppressed in the synchronous detection output signal by applying reset pulses to the reset gate electrode at times preceding admission of charge packets under the floating diffusion. Reset pulses precede charge packet admissions by time intervals longer than the reciprocal of the high-pass filter corner frequency in radians per unit time. This other way of keeping reset noise low is favored by many engineers, since the amplitude of reset pulses need not be so well controlled, as long as it is large enough to guarantee relatively low-impedance clamp of the floating diffusion to the reset drain diffusion during the duration of each reset pulse.
The modulation transfer factor (MTF) of a CCD rolls off at higher frequencies. To obtain flat response from the CCD, it has been common practice, particularly in the CCD camera art, to peak the high frequencies. Peaking is done by cascading after the CCD a linear-phase amplifier with gain boosted at high frequencies to compensate for the roll-off of high frequencies in the MTF of the CCD. Peaking increases the high frequency noise originally in the CCD and in elements of the amplifier, increasing it by the same factor as signal. It would be more desirable if the high-frequency roll-off of CCD MTF could be corrected boosting high-frequency signal by a factor larger than that by which high-frequency noise is increased.
Davy in his U.S. Pat. No. 4,330,753 describes the DSB AM sidebands seperated from the CCD imager output signal by band-pass filtering being applied to a balanced modulator to be synchronously detected. The balanced modulator demodulates the DSB AM sidebands at the harmonic of the clocking frequency to recover a replica of the original baseband signal content of the CCD output samples. Davy also discloses that synchronous detection can be made to show response for selected component spectra of the complete CCD output sample frequency spectrum by establishing a proper relationship between (a) the duty factor of the output samples from the CCD and (b) which of the clocking frequency harmonics is chosen for synchronous detection. Davy uses synchronous detection to suppress response to all components of the CCD output signal, except for one selected harmonic spectrum component of that CCD output signal. Response to the baseband component of the CCD output signal is suppressed in the output response of the balanced modulator used for synchronous detection; that is, feedthrough of the baseband component to synchronous detector output is prevented by a cancellation of input signal in the balanced modulator output circuit.
Supposing one modified Davy's apparatus to synchronously detect using a demodulator that is not balanced with regard to the sidebands being demodulated, the demodulator could feed additional components of the CCD output samples through to its output, were it not for the suppression of these other components by the band-pass filter. For example, supposing synchronous detection to proceed at the first harmonic of CCD clocking frequency (i.e., at that frequency itself), broadening the band-pass filter bandwidth would cause the upper frequencies of the baseband to feed through to the synchronous detector output. (A portion of the lower sideband of the second harmonic of carrier frequency could also feed through to the synchronous detector output. But the energy in it would be relatively small in most instances, owing to MTF roll-off, and would not be heterodyned to baseband.)
The additional upper frequency components that feed through the synchronous detector augment in a scalar addition process the upper frequencies recovered from the first harmonic sidebands by synchronous detection. Application of this general principle is made, for example, in the designing of stereophonic FM radio receivers of the time-division de-multiplexer type, in order to combine information in the baseband and in the stereophonic subchannel. What is of interest in the context of the invention presently being described is the effect feedthrough of the baseband spectrum through the synchronous detector has with regard to random noise arising in the imager electrometer MISFET and in the amplifier circuitry between the imager and the synchronous detector.
The high-frequency noise from baseband feedthrough is uncorrelated with high-frequency noise from synchronous detection at clocking frequency of the remainder of the signal, for all frequencies except half and three-halves clocking frequency. But noise at half and three-halves clocking frequency is above the band of interest in the processed CCD output signal. So then, scalar augmentation of the upper frequencies of CCD response, obtained by synchronous detection at clocking frequencies, is accompanied by only vectorially summed upper frequency noise components from the baseband and synchronously detected clocking-frequency sidebands. Accordingly, raising high-frequency response by the augmentation process disclosed in this specification provides up to a 3 dB better signal-to-noise ratio in the higher frequencies than is obtainable by conventional peaking.
Rather than using a band-pass filter to select the components of CCD output sample spectrum to be synchronously detected, one may arrange to trap energy from the CCD output sample spectrum prior to the synchronous detection process. This is advantageously done using a high-pass, low-reject filter for the lower frequencies of the spectrum below the double sideband spectrum being synchronous detected in its entirety at a carrier frequency harmonic.
This high pass filter can be a simple RC network. Where a floating diffusion electrometer with low-impedance clamp to reset drain during read is used, the RC high-pass filter is preferable to a band-pass filter before the synchronous detector. The band pass filter has a tendency towards ringing, which interferes with suppressing reset noise.